The present invention generally concerns plug filter assemblies, and more specifically expansion plug filter assemblies for communicating pressurized air from an axle spindle to a rotary union assembly. The expansion plug filter assembly of the preferred embodiment is inserted into the axle spindle, wherein the assembly is sealably secured against the interior wall of the axle spindle by an external o-ring to form a first seal and by an expandable press plug assembly that provides a second seal.